FIG. 9 is a constructing view of a general magnetron drive power-supply unit having a choke coil, a power element, a booster transformer, and the like. In the figure, alternating current from a commercial power-supply 11 is rectified to direct current by a rectifying circuit 13, the rectified current is smoothed by a choke coil 14 and a filter capacitor 15 which is provided at the output side of the rectifying circuit 13, and the smoothed current is applied to input side of an inverter 16. Direct current is converted to the desired high frequency (20 to 40 kHz) by on/off of a semiconductor-switching element in the inverter 16. The inverter 16 includes two groups of switching element groups for switching direct current with high speed where plural power MOSFET are connected in parallel for example, and a drive circuit driving these switching element groups. The drains of the power MOSFET constituting the switching element groups are connected to one end and the other end of a primary winding 181 of a booster transformer 18 respectively, both sources of the power MOSFETs constituting these two switching groups are connected, and further gates of the power MOSFETs constituting the switching groups are connected to the switching element drive circuit respectively. The switching element groups constituted by the power MOSFETs are driven by an inverter control circuit 161, and current flowing through primary side of the booster transformer 18 is switched on/off with high speed.
The primary side current of the rectifying circuit 13 is detected by a CT 17 as an input signal of the control circuit 161, the detected current is inputted to the control circuit 161 so as to use for control of the inverter 16.
The choke coil 14 and the filter capacitor 15 perform a function that a high frequency noise does not transmit to the commercial power-supply 11 while converting to high frequency from direct current, therefore, the choke coil 14 and the filter capacitor 15 have inductance and capacitance of a degree to remove the high frequency noise.
In the case that the core of the choke coil 14 is small, large inrush current flows causing the coil to saturate thus the choke cannot perform its function. In contrast, in the case that a large core is used, the coil does not saturate, however a disadvantage is that the choke coil itself becomes large in size and weight and therefore becomes heavy. Then, the problem of saturation is solved by using a choke coil winding a matter having a slit to longitudinal direction of a tube-shaped core as shown in FIG. 8 with electric wire.
In the booster transformer 18, a high frequency voltage being output of the inverter 16 is applied to the primary winding 181, and high voltage corresponding to turn ratio is obtained at a secondary winding 182. A winding 183 little in number of turns is provided at the secondary side of the booster transformer 18, and is used for heating a filament 121 of the magnetron 12. At the side of the secondary winding 182 of the booster transformer 18, a voltage doubler half-wave rectifying circuit 19 is provided to rectify the output of the booster transformer 18. The voltage doubler half-wave rectifying circuit 19 includes a high voltage capacitor 191 and two high voltage diodes 192 and 193, the high voltage capacitor 191 and the high voltage diode 192 are conductive at positive cycles (an upper end of the secondary winding 182 is positive for example in the figure), and electric charge is charged positive to the left side of the electrode plate of the high voltage capacitor 191 in the figure, negative to the right side of the electrode plate. Next, at negative cycles (a lower end of the secondary winding 182 is positive), the high voltage diode 193 is conductive, and doubler voltage plus voltage of the high voltage capacitor 191 previously charged and the secondary winding 182 is applied between an anode 122 and a cathode 121 of the magnetron 12.
FIG. 7 shows an example of mounting the conventional power-supply unit for electronic oven on a printing board. The mounting example shows that the booster transformer 20′ supplying power to a magnetron (not shown), the CT, the choke coil 30′, and a heat radiation fin 60′ are provided on the printing board 80.
Thus, in the conventional unit, a current transformer CT (17 in FIG. 9) detecting the primary side current of the rectifying circuit 13 (FIG. 9) is provided on the board. The turn ratio of the CT is about 1:2000, current flowing through one wind can be detected with minute current of 1/2000, and the CT transmits to the inverter control circuit with low loss.
The choke coil 30′ is provided on the board.
FIGS. 8A to 8C are views explaining the choke coil 30′, FIG. 8A is a plane view, FIG. 8B is a front view, and FIG. 8C is a perspective view. In the figures, symbol 31 is a cylindrical core constructed by high permeability material (ferrite material for example). Symbol 32 is a winding wound a large numbers of windings over the inside and outside of the cylindrical core 31, symbol 33 is a resin covering the whole cylindrical core 31, and the cylindrical core 31 and the winding 32 are insulated by the resin. Symbol 34 is an air gap for depressing saturation because the high permeability material such as ferrite and the like saturates rapidly at large current.
Returning to FIG. 7, the booster transformer 20′ provided on the board shows an example of a conventional booster transformer using a ferrite core. In FIG. 7, a primary winding 201, a secondary winding 202, and a heater winding 203 are arranged in parallel on the same axis of two facing horseshoe-shaped ferrite cores 204 and 205. In the case of power-supply for magnetron drive dealing frequently large power, zero volt switching system (ZVS system hereafter) by voltage resonance prevails. In the ZVS system, it is necessary to set the coupling coefficient of the booster transformer at about 0.6 to 0.85 to obtain resonance voltage, therefore, an air gap G is provided. Thus, the booster transformer 20′ converts low voltage applied to the primary winding 201 to high voltage corresponding to the turn ratio to generate at the secondary winding 202 without saturation even at large current.
The heat radiation fin 60′ is provided with a power semiconductor element at the state of a package P in which the power semiconductor element is covered with a molding resin. By providing the heat radiation fin 60′, heat generated at the power semiconductor element is transmitted to the heat radiation fin 60′ through the package P, the power semiconductor element is not heated because the heat is diffused efficiently from here, therefore heat failure of the power semiconductor element does not generate.
Since the CT is large, that is the turn ratio thereof is 1:2000 as mentioned above, the amount of space the CT occupies on the board cannot be neglected.
In the case of the prior booster transformer using two facing horseshoe-shaped ferrite cores 204 and 205, it is necessary to increase peak current flowing through the primary side of the booster transformer in order to make the output of the magnetron higher in the booster transformer 20′. By that, the ferrite core becomes easy to saturate because saturation magnetic flux density characteristic is bad, and a large ferrite core is necessary for no or little saturation. This is an obstruction of the miniaturization of the power-supply.
Since the heat radiation fin 60′ is in the state of the package P in which the power semiconductor element is covered with mold resin, heat generated at the power semiconductor element is transmitted to the heat radiation fin 60′ through the package P so that heat radiation performance is not entirely good.
To solve the problem, the invention provides a power-supply unit for an electronic oven enabling to detect current without using the CT and realizing miniaturization, lightweight, and low cost by miniaturizing of the booster transformer and by the small size of the heat radiation fin in which cooling efficiency of the power semiconductor element high.